Removal of metronidazole from wastewater by electrocoagulation with chloride ions electrolyte: The role of reactive chlorine species and process optimization

Electrocoagulation (EC) has been attracting increasing amounts of attention due to its ability to remove the pollutants in widespread wastewater. Chloride ions (Cl-) are the most commonly used electrolyte in the EC process. However, the role of the reactive chlorine species (RCS) generated near the electrode is often being underemphasised. In this study, the experiments focused on the generation of RCS and its contribution to the removal of metronidazole (MNZ) during EC. The present findings demonstrated the presence of Cl. and ClO. near the anode in solution, which dominated the degradation of MNZ. The MNZ decomposition pathways was proposed based on the generation of intermediate products. The toxicity of MNZ and its main degradation products was evaluated by Toxicity Estimation Software Tool (TEST) model and most of intermediates were less toxic than MNZ. Furthermore, the flocs could adsorb part of MNZ by characterizing the flocs using SEM-EDS, FT-IR, XRD and XPS. The contribution ratio of the flocs adsorption and the RCS oxidation for removing MNZ were 57.30% and 41.70%, respectively. Response surface methodology (RSM) was applied to optimize the operation parameters. The present work reveals a new mechanism of EC and manifest good potential for removing antibiotics from chloride-containing wastewater.